Golf club head and method of manufacturing the same

ABSTRACT

A golf club head comprises a face portion formed by: forging a beta type titanium alloy while keeping the temperature of the alloy lower than the lower critical solution temperature of the alloy; and aging the forged alloy under the lower critical solution temperature Tc so as not to become a solution state. A method of manufacturing a golf club head comprises: preparing a billet of a beta type titanium alloy; forging the billet into the face member while keeping the temperature of the billet lower than the lower critical solution temperature Tc of the beta type titanium alloy; and aging the face member under the lower critical solution temperature Tc so as not to become a solution state.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a golf club head and a method ofmanufacturing the same, more particularly to a thermal treatmentincorporated into a plastic forming process which can improving therestitution coefficient.

[0002] In recent years, in order to improve the restitution coefficientof the golf club heads, beta type titanium alloys are used to form theclub face. In general, the beta type titanium alloys are low in theYoung's modulus in comparison with alpha and alpha-beta type titaniumalloys, and it is considered as being easier to make the face portionhaving a good restitution coefficient.

[0003] In case the face portion is shaped from a beta type titaniumalloy through hot forging, if the hot forging is carried out as usual ata temperature of about 20 to 50 deg.C higher than the lower criticalsolution temperature of the alloy, the forging is provided with a singlephase of platy beta crystal structure as shown in FIG. 5(a). Then, ifthe forging is subjected to an aging treatment, microscopic granularalpha crystals occur in the platy beta crystal as shown in FIG. 5(b).This structure is superior to the others in respect of the strength andhardness, but the elongation percentage is vary small. Thus, this isbreakable. In order to cover this breakable nature, if the thickness ofthe face material is increased, the durability may be increased but itbecomes difficult to get a good restitution coefficient because therigidity is also increased.

SUMMARY OF THE INVENTION

[0004] It is therefore, an object of the present invention to provide agolf club head and a method of manufacturing the same, in which therestitution coefficient and durability can be improved to a higherdimension in a balanced manner.

[0005] According to one aspect of the present invention, a golf clubhead comprises a face portion whose front face defines a club face forhitting a ball, wherein the face portion is formed by: forging a betatype titanium alloy while keeping the temperature of the alloy lowerthan the lower critical solution temperature of the alloy; and aging theforged alloy under the lower critical solution temperature so as not tobecome a solution state.

[0006] Here, the lower critical solution temperature means the lowerlimit of the temperature range within which the alloy becomes platy betaphase as time goes by.

[0007] According to another aspect of the present invention, a method ofmanufacturing a golf club head which comprises a main body and a facemember attached thereto, comprises:

[0008] preparing a billet of a beta type titanium alloy;

[0009] forging the billet into the face member while keeping thetemperature of the billet lower than the lower critical solutiontemperature of the beta type titanium alloy; and

[0010] aging the face member under the lower critical solutiontemperature so as not to become a solution state.

[0011] Here, the “billet” means not only that produced by casting butalso by another method such as extrusion and forging.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a front view of a golf club head according to thepresent invention.

[0013]FIG. 2 is a cross sectional view thereof taken along a line X-X inFIG. 1.

[0014]FIG. 3 is an exploded perspective view showing an example of thestructure of the head.

[0015] FIGS. 4(a) and 4(b) are diagrams for explaining the change in thecrystallographic structure of a beta type titanium alloy according tothe present invention.

[0016] FIGS. 5(a) and 5(b) are diagrams for explaining the change in thecrystallographic structure of a beta type titanium alloy according tothe conventional temperature control.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Embodiments of the present invention will now be described indetail in conjunction with the accompanying drawings.

[0018] In the drawings, club head 1 according to the present inventionis a metal wood-type hollow golf club head comprising a face portion 2whose front face defines a club face F for hitting a ball, a crownportion 3 intersecting the club face F at the upper edge 2 a thereof, asole portion 4 intersecting the club face F at the lower edge 2 bthereof, a side portion 5 between the crown portion 3 and sole portion 4which extends from a toe-side edge 2 t to a heel-side edge 2 e of theclub face F through the back face of the club head, and a neck portion 6to be attached to an end of a club shaft (not shown). The neck portion 6is provided with a shaft hole 6 a into which a club shaft is insertedand fixed with an adhesive agent or the like.

[0019] In FIG. 1 and FIG. 2, the club head 1 is in its address position,namely, the club head 1 is set on a horizontal plane HP such that thecenter line CL of the clubs haft is inclined at the lie angle alphawhile keeping the center line CL on a vertical plane, and the club faceF forms its loft angle beta with respect to the horizontal plane HP.Incidentally, when setting the club head 1 alone in its addressposition, the center line of the shaft inserting hole 6 a may be usedinstead of the center line CL of the clubs haft.

[0020] In order to widen the area of the face portion which area candeform at impact and contribute to improving of the restitutioncoefficient, the size of the wood-type club head is designed as follows.

[0021] The volume of the club head 1 is preferably set in a range of notless than 300 cc, more preferably 300 to 500 cc, still more preferably310 to 450 cc. The horizontal width B of the club face F between itstoe-side extreme end and heel-side extreme end is preferably set in arange of not less than 90 mm, more preferably not less than 95 mm, butnot more than 130 mm, more preferably not more than 115 mm. The height(A) of the club face F between its upper end and lower end is preferablyset in a range of not less than 45 mm, more preferably not less than 48mm, still more preferably not less than 50 mm, but not more than 70 mmwhen measured along a direction inclined at the loft angle beta as shownin FIG. 2. The aspect ratio (A/B) of the height (A) to the width (B) ispreferably set in a range of from 0.346 to 0.667. If the face height (A)is more than 70 mm and/or the face width (B) is more than 130 mm and/orthe aspect ratio (A/B) is more than 0.667, then the deformation atimpact becomes too large and the durability will be decreased rapidly.If the face height (A) is less than 45 mm and/or the aspect ratio (A/B)is less than 0.346, then the area becomes insufficient, and it becomesdifficult to improve the restitution coefficient. If the face width (B)is less than 90 mm, as the deformation at impact becomes less, theimprovement in the restitution coefficient is decreased.

[0022] In this embodiment, as shown in FIG. 2, the face portion 2 isprovided along the circumferential edge thereof with a comparativelythin peripheral region 2B. Accordingly, the face portion 2 is made up ofthe thin peripheral region 26 and a thicker central region 2A includingthe sweet spot and surrounded by the thin peripheral region 26. Thethickness t1 of the thick central region 2A is preferably set in a rangeof not less than 1.8 mm, more preferably not less than 2.1 mm, but notmore than 2.9 mm. The thickness t2 of the thin peripheral region 26 ispreferably set in a range of not less than 1.0 mm, more preferably notless than 1.3 mm, but not more than 2.8 mm. The thickness difference(t1−t2) is set in a range of less than 1.9 mm, preferably less than 1.5mm, but more than 0.1 mm, preferably more than 0.2 mm. The thinperipheral region 26 has a groove width more than about 2 or 3 mm. Suchthickness arrangement may further improve the restitution coefficient ofthe club face F without decreasing the strength and durability of theface portion 2.

[0023]FIG. 3 shows a two-piece structure which is suitably employed inthe above-mentioned metal wood-type hollow golf club head 1. Thistwo-piece structure comprises an open-front hollow main body 9 and aface member 7 welded to the front of the main body 9 to form the clubface F. The face member 7 is made up of a platy main portion 7A definingsubstantially the entirety of the club face F and an extension 7Bextending backward from the edge (2 a, 2 b, 2 t, 2 h) of the mainportion 7A. In this example, the extension 7B includes a crown-sideextension 7B1 extending from the entire length of the upper edge 2 aforming a front part of the crown portion 3, and a sole-side extension7B2 extending from the entire length of the lower edge 2 b forming afront part of the sole portion 4. But, an extension is not provided atthe toe-side edge 2 t and heel-side edge 2 h. The head main body 9 inthis example is therefore, made up of the remaining part of the crownportion 3, the remaining part of the sole portion 4, the side portion 5,and the neck portion 6. The front edge of the head main body 9 is shapedto fit to the face member 7, more specifically, set back in a portioncorresponding to the extension 7B. The depth S of the extension (7B1,7B2) which is defined as the shortest distance from any point at therear edge 7 e to the club face edge (2 a, 2 b) is preferably set in arange of from 5 to 20 mm, more preferably 5 to 15 mm.

[0024] The face member 7 is shaped from a billet of a beta type titaniumalloy through hot forging at a temperature lower than the lower criticalsolution temperature Tc of the alloy. For example, Ti—15V—3Cr—3Al—3Sn,Ti—15Mo—5Zr—3Al, Ti—13V—11Cr—3Al, Ti—8Mo—8V—2Fe—3Al, Ti—22V—4Al,Ti—15Mo—5Zr and the like can be used. The billet of the beta typetitanium alloy has a fibrous beta crystal structure as shown in FIG.4(a). Here, the “billet” means not only that produced by casting butalso by another method such as extrusion and forging. The billet in thisexample is a round bar having a diameter of about 20 mm.

[0025] As mentioned above, the lower critical solution temperature Tccorresponds to the lower limit of the temperature range within which thealloy becomes solution and as time goes by platy beta phase crystal isproduced. In other words, Tc corresponds to the upper limit of thetemperature range within which platy beta phase crystal is neverproduced. This temperature Tc may be varied depending on the componentsof the alloy but determined uniquely to the components. For example, Tcof Ti—15V—3Cr—3Al—3Sn is 760 deg.C, and Tc of Ti—15Mo—5Zr—3Al is 780deg.C.

[0026] The forging temperature at which the hot forging is carried outis therefore, set to be lower than the lower critical solutiontemperature Tc deg.C, preferably not less than (Tc-100 deg.C), morepreferably not less than (Tc-50 deg.C), but preferably not more than(Tc-20 deg.C). Further, the forging temperature is set to be not lessthan the recrystallization temperature of the beta type titanium alloy.Therefore, due to stress during forging and the forging temperaturedecreased to under Tc, the fibrous beta crystal structure can bemaintained after forging as show in FIG. 4(a).

[0027] If the forging temperature decreased to lower than (Tc-100)deg.C, it becomes hard for the alloy to cause plastic flow, and as theworkability deteriorates, a costly high-power forging machine will benecessitated. Further, during forming, cracks and breakage become liableto occur. By setting the forging temperature less than (Tc-20) deg.C,the alloy is prevented from becoming solution without fail.

[0028] In this invention, the hot forging means a process of forming thematerial billet into a specific shape utilizing its compressive plasticdeformation caused by hitting and/or pressing while heating up thematerial to the above-mentioned specific temperature. For example, socalled free forging, open die forging, closed die forging, semi-closeddie forging, high-speed forging, isothermal forging and the like areincluded.

[0029] If the surface of the forging or face member 7 is covered withunwanted oxidized film after the hot forging, it is removed by polishingor the like. In view of prevention of scales, therefore, closed dieforging is preferred. In view of production efficiency, on the otherhand, two or three step forging where preforming and finish up formingand optional intermediate second forming are carried out changing diesis preferred.

[0030] In this embodiment, the main body 9 is a precision casting of ametal material formed by lost-wax process. For the main body, variousmaterials may be used, for example, the same material as the face membermay be used. In this example, however, a high-strength titanium alloyTi—6Al—4V which is different from the beta type titanium alloy of theface portion 2 is used.

[0031] The face member 7 which is made through the forging as explainedabove is welded to the head main body 9. Then, an aging treatment iscarried out on the face member 7 together with the main body 9, in otherwords, carried out on the entirety of the club head, while preventingthe alloy from becoming solution.

[0032] The aging treatment is thus such that the object is kept at atemperature of from Tc-220 to Tc-130 deg.C (specifically 550 to 650deg.C in this example) for a predetermined time period of from 4 to 20hours, and subsequently it is air cooled at a room temperature, namely,slow cooled.

[0033] Therefore, during aging, microscopic granular alpha crystalsoccur in the fibrous beta crystal structure as shown in FIG. 4(b),whereby the strength becomes somewhat lower if compared with thestructure show in FIG. 5(b) but maintains an improved sufficient level,and the decrease in the elongation can be effectively prevented.

[0034] If quenching or rapid cooling such as water-cooling is made onthe face portion, the internal stress and/or internal strain occuredduring forging remains and the strength is decreased.

[0035] The aging treatment can free the strain and stress from thewelded part and its vicinity as well and thereby the weld junction canbe improved in the durability. If the welding is made after the agingtreatment, the welding region becomes a single-phase of platy betacrystal and the strength will be decreased.

[0036] The above-mentioned extension 7B can shift the weld bead (w) at adistance from the circumferential edge of the club face F. Therefore,the formation of a rigid part near the face portion 2 which may hinderthe improvement of the restitution coefficient can be avoided.

[0037] Further, during welding, the heat transfer from the welding partto the club face can be decreased or dispersed by the extension 7B, andaccordingly, the undesirable change in the crystallographic structure tothe platy beta crystal of the face portion can be effectively prevented.

[0038] Comparison Tests

[0039] Wood-type golf club heads were made by welding various facemembers to identical main bodies. All the main bodies were a precisioncasting of Ti—6Al—4V having the structure shown in FIG. 3. The facemembers had an identical shape shown in FIG. 3 but they were madechanging the conditions of the forging and aging treatment as shown inTable 1, using two materials:Ti—15V—3Cr—3Al—3Sn and Ti—15Mo—5Zr—3Al. Theclub heads were then tested for the restitution coefficient anddurability, and the tensile strength and elongation at rupture of theface member materials were measured as follows.

[0040] Strength and Elongation of Face Member Materials:

[0041] To determine the tensile strength and elongation at rupture, aplaty 3 mm thick test piece shaped from a round bar by hot forging underthe same conditions as each face member was used, and these parameterswere measured using a tensile tester.

[0042] Restitution Coefficient:

[0043] According to the “Procedure for Measuring the velocity Ratio of aclub Head for conformance to Rule 4-1e, Appendix II, Revision 2 (Feb. 8,1999), United states Golf Association.”, the restitution coefficient “e”was obtained.

[0044] Durability:

[0045] The club heads were attached to identical shafts to make woodclubs. Each club was attached to a swing robot and hit golf balls at ahead speed of 50 m/s repeatedly until the face portion was broken. Thenumber of hits is shown in Table 1. If the number of hits reached to5000 without break, the head is appraised as passable and indicated as“OK” in Table 1.

[0046] Production Efficiency in Forging Process:

[0047] With respect to each face member, ten pieces were made and theirdimensional variations were evaluated into three ranks (A, B and C) asfollows.

[0048] A: All the ten pieces had dimensions as designed.

[0049] B: Four to nine pieces had dimensions as designed.

[0050] (The remainder needed adjustments by machine work)

[0051] C: Three or less pieces had dimensions as designed.

[0052] (The remainder needed adjustments by machine work)

[0053] From the test results, it was confirmed that the club headsaccording to the present invention can be improved in the restitutioncoefficient and durability in a balanced manner. TABLE 1 Club head Ex. 1Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ref. 1 Ref. 2 Ex. 6 Ex. 7 Ex. 8 Face memberMaterial *1 T1 T1 T1 T1 T1 T1 T1 T2 T2 T2 Lower critical solution 780780 780 780 780 780 780 760 760 760 temperature Tc (deg. C.) Forgingtemperature 750 760 690 660 730 800 800 730 740 680 (deg. C.) Agingtreatment Temperature (deg. C.) 600 600 600 600 600 480 600 570 570 570Time (Hour) 5 5 5 5 5 10 5 5 5 5 Face center thickness t1 2.75 2.75 2.752.75 2.75 2.75 2.75 2.75 2.75 2.75 (mm) Tensile strength (MPa) 1430 14121489 1466 1440 1780 1988 1330 1321 1365 Elongation at rupture (%) 7.5 87.2 7.1 7.4 3.1 1.9 8.5 9.1 8.5 Restitution coefficient 0.853 0.86 0.8570.858 0.854 0.855 0.85 0.85 0.849 0.852 Durability OK OK OK OK OK 22001500 OK OK OK Production efficiency A A B C A A A A A B Club head Ex. 9Ex. 10 Ref. 3 Ref. 4 Face member Material *1 T2 T2 T2 T2 Lower criticalsolution 760 760 760 760 temperature Tc (deg. C.) Forging temperature650 710 800 800 (deg. C.) Aging treatment Temperature 570 570 480 570Time (hour) 5 5 10 5 Face center thickness t1 2.75 2.75 2.75 2.75 (mm)Tensile strength (MPa) 1344 1336 1650 1898 Elongation at rupture (%) 8.28.6 4.9 2.1 Restitution coefficient 0.848 0.85 0.852 0.845 Durability OKOK 2500 1800 Production efficiency C A A A

1. A golf club head comprising a face portion whose front face defines aclub face for hitting a ball, wherein the face portion is formed by:forging a beta type titanium alloy while keeping the temperature of thealloy lower than the lower critical solution temperature of the alloy;and aging the forged alloy under the lower critical solution temperatureTc so as not to become a solution state.
 2. A method of manufacturing agolf club head, the golf club head comprising a face member comprising aface portion whose front face defines a club face for hitting a ball,and a main body to which the face member is attached, the methodcomprising: preparing a billet of a beta type titanium alloy; forgingthe billet into the face member while keeping the temperature of thebillet lower than the lower critical solution temperature Tc of the betatype titanium alloy; and aging the face member under the lower criticalsolution temperature Tc so as not to become a solution state.
 3. A golfclub head according to claim 1, wherein the temperature of the hotforging is not less than Tc-50 deg.C and not more than Tc-20 deg.C.
 4. Agolf club head according to claim 1 or 2, wherein the face portion isprovided with a backward extension, the backward extension extendingfrom an edge of the face portion and formed integrally with the faceportion through said forging, and the backward extension is welded to amain body of the head which is formed by casting.
 5. A method accordingto claim 2, wherein the temperature of the hot forging is not less thanTc-50 deg.C and not more than Tc-20 deg.C.
 6. A method according toclaim 2, which further comprises casting the main body.
 7. A methodaccording to claim 2, wherein the face member further comprises abackward extension extending from an edge of the face portion to bewelded to the main body.
 8. A method according to claim 2, 5, 6 or 7,which further comprises welding the face member to the main body beforeaging the face member.